This invention relates to the electrical interconnection of electronic components in general and more specifically to a method for wire bonding electronic components to flexible printed wiring boards.
Flexible printed wiring boards or flex boards are generally used to transmit electrical signals between electronic devices that are moveable with respect to one another. For example, flex boards may be used to transmit electrical signals from the central processing board or xe2x80x9cmother boardxe2x80x9d of a laptop computer to the display screen. A typical flex board may comprise one or more layers of metal circuit paths or conductors that are bonded onto a thin, flexible, insulating substrate. The circuit paths or conductors are commonly thin copper strips and the flexible, insulating substrate typically comprises a polyimide material, although other materials may be used. Depending on the complexity of the electronic circuit, the flex board may be single sided, double sided, or may comprise multiple layers of circuit paths or conductors, as in a multi-layer flex board.
Traditionally, flex boards of the type described above did not include any electronic components and merely served as a means to interconnect the electronic components mounted to various rigid printed wiring boards. However, recent developments in flex board technology have allowed such flex boards to support some kinds of electronic components, thus allowing them to be substituted for conventional rigid boards in certain types of circuits. Of course, the ability to replace conventional rigid printed wiring boards with flex boards creates many new design opportunities for electronic devices.
One common method of mounting electronic components, such as integrated circuits (ICs), to flex boards is to solder them directly to the copper conductors on the flex board; the soldered connection providing not only the required electrical connection, but mechanical support as well. Unfortunately, however, the soldering process is not without its disadvantages, and limitations on the maximum density of electrical connections limits the maximum miniaturization that can be achieved with soldering.
One technology that does allow for high density electrical connections is wire bonding. While several variations exist, the basic wire bonding process begins by firmly attaching the back side of the electronic component, usually an integrated circuit (IC), onto the substrate using either an organic adhesive, a glass, or a metal-alloy reflow process. Once the IC is attached to the substrate, it is electrically connected to the various circuit conductors on the board by a plurality of fine wires. The wires are bonded or welded, one at a time, to the pads on the IC and on the board using a special tool, such as a wedge or a capillary, and a combination of heat, pressure, and/or ultrasonic energy.
Although wire bonding to rigid boards was originally done manually, with the operator""s skill controlling every aspect of the bonding process, it has progressed rapidly to a fully automated process as the density of IC interconnections has increased. In the automated wire bonding process, an automatic wire bonding machine senses the locations of the bonding pads on both the IC and the board and then automatically connects the appropriate pads with the fine wire. Such automated wire bonding processes are well-developed and have kept pace with the ever increasing number of connections required as well as shrinking bonding pad size. For example, it is not uncommon to bond ICs requiring 300 connections and having two rows of alternating perimeter bonding pads with pad sizes as small as 50xc3x9750 xcexcm (2xc3x972 mils) with 100 xcexcm (4 mils) between on-row pad centers.
Obviously, such high density wire bonding requires very accurate placement of the IC on the board and requires that the board itself be accurately aligned with respect to the wire bonding machine. While such problems have been addressed and solved with respect to rigid substrates or boards, considerable problems remain with respect to flex boards. For example, flex boards by their nature provide little mechanical support for the electronic components, which makes it difficult to securely hold the flex board with respect to the wire bonding machine during the wire bonding process. Another problem is that the thin flex boards tend to lift or wrinkle when placed on a flat surface, thus making it extremely difficult, if not impossible, for the wire bonding machine to make the necessary wire bond connections.
One method that has been tried to securely hold the flex board in fixed relation to the automated wire bonding machine, and to hold it flat, has been to use double sided adhesive tape to secure the back side of the flex board to a suitable flat support platen. While the double sided tape solves the problem of securely holding the flex board flat with respect to the wire bonding machine, it is difficult to remove the double sided adhesive tape after the wire bonding process without damaging the delicate wire bonds or the flex board itself. Also, such a process does not readily lend itself to automated production techniques.
Consequently, there remains a need for an apparatus and a method for electrically connecting electronic components to a thin, flexible wiring board. While it would be preferable to use a wire bonding process to electrically connect the components to the board, the problems heretofore associated with accurately locating the flex board with respect to the wire bonding machine, as well as holding it flat, must be solved.
A method of electrically connecting an electronic component to the flexible printed wiring board may comprise the steps of: Providing an air permeable platen having a substantially flat front surface and a back surface; placing the flexible printed wiring board on the front surface of the platen; lowering the pressure at the back surface of the platen; and electrically connecting the electronic component to the flexible printed wiring board.
A significant advantage of the present invention is that it provides a means for securely holding the flex board with respect to the wire bonding machine without the need to resort to adhesives, with all their associated disadvantages. Another advantage is that the air permeable platen provides a solid support surface for the flex board while at the same time allowing more evenly distributed external air pressure to hold the flex board flat against the front surface of the air permeable platen.